Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

For the optical characterization of the CIGS solar cells with regard to optoelectronic simulations at
ZHAW, ellipsometry measurements were performed. The individual layers were deposited on silicon
wafers or glass substrates and measured by variable angle spectroscopic ellipsometry (VASE®). The
samples were measured in two batches. While the results of the first batch are already available
([Ga]/[In+Ga] = 0.3 CIGS, TCOs, Mo), we are still waiting for the results from the second batch (CIGS
with varying Ga content, CdS).
High mobility transparent conducting oxide
In a DSC/CIGS tandem device much light is lost in the infrared region, i.e. cannot be converted by the
bottom cell, due to absorption by free charge carriers in the transparent conducting oxide (TCO) used
in the top cell. Commonly fluorine doped tin oxide (FTO) is used in standard DSCs, but TCOs with
high electron mobility, such as titanium doped indium oxide (ITiO), show superior optical transmittance
and electrical conductivity.
Both the front and back electrodes of the DSC were replaced by ITiO coated glass. The commonly
used back electrode can easily be substituted by ITiO, leading to a substantial increase in transmittance
in the infrared. Replacing the front contact by ITiO is more demanding. A thin TiO2 film must be
deposited on the front electrode and sintered at high temperatures. With the standard deposition procedure
the TiO2 film does not adhere well to the smooth ITiO surface. In addition, the sheet resistance
increases tenfold when sintering the TiO2 film at high temperatures under ambient conditions. So far
6.7% power conversion efficiency was reached with a DSC using an ITiO front contact.
Optimized stacked hybrid tandem solar cells
The record conversion efficiency for a stacked DSC/CIGS tandem is currently still at 15% as reported
by Brémaud and Liska et al. in 2006. So far it was not possible to make stacks with higher efficiencies
because of performance limitations of the top cell (low photocurrent and low transmittance). With the
new materials now available, i.e high performance dye and highly transparent ITiO, we expect a substantial
increase in performance of stacked tandem devices.
In parallel, solid organic solar cells to be used in hybrid or non hybrid multijunction devices were further
ameliorated at Empa. The material class of polymethine dyes (in particular photographic cyanine
dyes) which was already used at the starting point of the project was still in the focus of the activities.
By partially oxidizing one of the active layers in the solar cell, power conversion efficiencies > 4% were
achieved. This is a promising result, given the fact that this organic solar cell absorbs only in a restricted
visible range from 350 nm to 610 nm. Future efforts will be directed towards extending photosensitivity
to the far red to near infrared domain. In this respect, novel squaraine dyes synthesized by
the Laboratory for Functional Polymers look very promising. These dyes were primarily designed to be
used in DSC cells, in particular for the construction of DSC/DSC tandem cells.
Optical in-coupling calculations performed at the Institute of Computational Physics at ZHAW provide
valuable assistance in evaluating the optimum device structure and to estimate the internal losses
from experimental data. In order to apply the above-described simulation approach to the other solar
cell types in this project the optical simulation of thicker (> 1 µm) solar cell structures has also been
perpetuated. Because of the thicker structures, incoherent optical simulations are employed. The numerical
calculation of photon absorption profiles for arbitrary coherent/incoherent layer structures is
currently being implemented. This will allow the numerical characterization of tandem solar cell structures
with matched currents for achieving higher efficiencies.
Development of monolithically integrated hybrid tandem solar cells
The monolithic integration of DSC/CIGS tandem cells could be demonstrated successfully for the first
time. A scheme of the device setup is shown in Figure 2. With this setup one TCO layer (the back
electrode of the top cell) can be omitted, reducing optical and electric losses.
ThinPV, F. Nüesch, Empa
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